Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic system or an electrostatic recording system, and more particularly to an image forming apparatus which includes an optical detection portion.
Description of the Related Art
Downsizing of the entire image forming apparatus has been a critical issue, and a known downsizing technique for a color image forming apparatus is that toner images of respective colors on an image bearing member are once transferred onto an intermediate transfer belt, and the toner images of a plurality of colors on the intermediate transfer belt are collectively transferred to a recording material.
Generally speaking, a toner image is transferred from the intermediate transfer belt to a transfer material by a transfer portion using an electrostatic transfer process, in which transfer bias (transfer voltage) is applied, and electrostatic attraction is generated so as to form an electric field having the reverse polarity of the charging polarity of the toner image. As a method of preventing transfer failure in such a transfer portion by leakage of the transfer bias, Japanese Laid-open Patent Publication No. 2009-128481 discloses a method of grounding using a static elimination circuit which can switch the resistance value of the transfer material transport guide.
This image forming apparatus also includes a color resist control portion. First a resist mark (toner mark) using the toner of each color is formed on the intermediate transfer belt as a reference image for resist detection, by forming the toner image on the photosensitive drum and transferring the toner image onto the intermediate transfer belt. Then the resist marks are detected by an optical sensor, which is installed on the downstream side of the black image (last color) forming portion of the intermediate transfer belt, and color resist control, such as correcting the image writing start position onto the photosensitive drum, is performed. The optical sensor is disposed to irradiate the light of the optical sensor to a position where the intermediate transfer belt is wound around the rollers, so that the intermediate transfer belt does not deviate in the surface direction causing a change in the distance between the optical sensor and the intermediate transfer belt. In such an optical sensor, Japanese Laid-open Patent Publication No. 2015-82065 discloses a method of disposing a conductive member, to attract the discharged current, near the optical sensor for grounding, to prevent an electrostatic breakdown caused by user access when the intermediate transfer belt is replaced or the transfer material is jammed.